Mechanical or electrical switching involves power losses. There are two types of losses in switching, switching losses and conduction losses. If the switch is extremely fast and conducts well in the on state, the switching losses are mainly associated with the switching time and attached circuits The conduction loss is determined by a switching module's on-state resistance after switching occurs and current is flowing through the switch terminals.
MOSFETs operate exceptionally well at low temperatures especially at cryogenic temperatures (below 200 K). FIG. 1 shows a dramatic drop, a reduction factor of 37.5, in on-state resistance from temperatures 400 K to 77 K. This leads to very low conduction losses at cryogenic temperatures, a very attractive feature for a switch. Switching losses, on the other hand, are not affected by temperature directly but by the switching speed of the electronic drive system. For example, FIG. 8, in the case of switching inductive loads, the current (Io) is constant during switching and the voltage across the switch falls linearly from Vo to nearly zero over some switching time τ. The switching energy dissipated in this transition is ½ VoIoτ. The dissipated power is the dissipated energy times the number of switching transitions per second. For two transitions per cycle, the dissipated power is VoIoτ/T, where T is the period of the switching cycle. In this instance, the switching losses depend only on the switch voltage, current and duty cycle (τ/T).
Switching losses can contribute a great deal of heating. Lowering the MOSFET module temperature increases the switching speed and reduces the intrinsic switching time, thus theoretically reducing the switching losses of the MOSFET switch. In practice, however, it is difficult to take advantage of these higher speeds. At low temperatures, there is a refrigeration penalty due to the switching loss heat load, which adds to the overall system losses. The reduction in switching losses due to a speed increase at lower temperatures usually does not offset the refrigeration penalty. With the increased speeds come other practical problems including ringing, etc. Many times, it is best to run the modules at moderate speeds, slower than that of the intrinsic module itself.